Unraveling Halogen Role in Two-Step Solution Growth of Organic–Inorganic Hybrid Mixed-Halide Perovskites: Guidelines of Fabricating Single-Phase Perovskites with Predictable Stoichiometry

The challenge faced in optoelectronic applications of halide perovskites is their degradation. Minimizing material imperfections is critical to averting cascade degradation processes. Identifying causes of such imperfections is, however, hindered by mystified growth processes and is particularly urgent for mixed-halide perovskites because of inhomogeneity in growth and phase segregation under stresses. To unravel two-step solution growth of MAPbBrxI3–x, we monitored the evolution of Br composition and found that the construction of perovskite lattice is contributed by iodine from PbI2 substrate and Br from MABr solution with a 1:1 ratio rather than a 2:1 ratio originally thought. Kinetic analysis based on a derived three-stage model extracted activation energies of perovskite construction and anion exchange. This model is applicable to the growth of PbI2 reacting with a mixed solution of MABr and MAI. Two guidelines of fabricating single-phase MAPbBrxI3–x with predictable stoichiometry thus developed help strategizing protocols to reproducibly fabricate mixed-halide perovskite films tailored to specific optoelectronic applications.

Preparation of PbI 2 film.Fused-silica substrates were undertaken ultrasonic cleaning sequentially in Aquet detergent, acetone and 2-propanol, followed by plasma treatment with 30010 mTorr of oxygen (99.5%) and 401 Watt in a plasma cleaner (PCD150, Allreal) for 5 min.The PbI2 precursor solution (1.2 M) was prepared by dissolving 600 mg PbI2 in DMF/DMSO mixture solvents (10:0.9 in volume ratio) at 70C and stirring at 500 rpm.The solution was cooled down to room temperature before use.Then 70 μl of the PbI2 precursor was spin-coated over the cleaned substrate with a spin speed of 4000 rpm for 30 sec.The precursor film was annealed at 70C for 10 min with a hotplate.The whole preparation procedure was performed in a nitrogen-purged glove box.

Preparation of MABr and MABr:MAI solution.
The MABr precursor solution was prepared by dissolving 40 mg of MABr in 20 ml of IPA, yielding a concentration of 18 mM.The mixed MAI/MABr precursor solutions were prepared with the different mole fraction of MABr,  Br , to obtain a precursor concentration of 18 mM.The precursor solutions were stirred at 500 rpm and heated at 50C overnight and then cooled down to room temperature before use.The whole preparation procedure was performed in a nitrogen-purged glove box.

Fabrication of perovskite.
The reaction chamber designed to carry out this study is detailed in its 3D drawing (Figure S1).It is composed of chamber cap and bottom.The bottom holds a dish to contain the precursor solution.The chamber with a cap and a bottom together enclosed all the precursors to maintain the MABr concentration with an equilibrated gas-liquid phase of IPA during the growth.The petri dish with the precursor solution was held by the chamber bottom that was heated by a hotplate underneath to provide a reaction temperature  R above room temperature but below the boiling point of IPA (82.3C). R , measured with a K-type thermocouple inserted into the chamber, was found to be different from the set temperature of the hotplate by less than 1C.A sample holder, fixed at the bottom end of a sample post, supports the placement of the sample which is realized by sliding the sample through the edge groves of the holder.The sample post, extended through a hole on the cap, is used to move the sample in and out of the precursor solution in the dish.
The reaction time was precisely controlled by pushing the sample post to immerse the sample holder in the precursor solution and by pulling it up to move the sample holder out of the solution.Mixed lead halide perovskite MAPbI3-xBrx films were fabricated using two reaction scenarios of the twostep solution-growth method.The chamber cap and bottom are made of aluminum, while the sample post and holder are all made of polyetheretherketone (PEEK).All the fabrication processes were also conducted in a nitrogen -purged glove box (H2O < 0.1 ppm, O2 < 10.0 ppm).

SI-2 Angular Calibration of XRD
The XRD angle  was calibrated by using the relationship between Bragg diffraction angle and its corresponding lattice constant a. Owing to the fact that the initial angular offset Δ of each sample in the sample stage is variable, the actual angle 2 of XRD was calibrated with the relationship between the observed first-and second-order diffraction peaks,  1 * and  2 * , respectively: where  XRD is the X-ray wavelength.With the above two equations, the lattice constant  along a specific crystal direction and the angular offset Δ were solved with the  1 * and  2 * extracted by Gaussian-fitting the first-and second-order diffraction peaks of the measured XRD profile.That is, for each XRD measurement, the procedure above was carried out to determine the actual XRD angle  (=  * + Δ) and the lattice constant of the sample.

SI-3 Extraction of Energy Bandgap
In this study, the bandgap energy  g was extracted from the absorption edge of the UV-vis absorption spectrum (), where E is photon energy, as depicted in Figure S2. g is taken as the intersect between the tangent line through the inflection point of the absorption edge part and the baseline.the absorption coefficient at 500 nm was estimated to be around 1.110 5 cm -1 , which is slightly lower than that obtained from single-crystal PbI2 (1.3810 5 cm -1 ). 2,3The difference may be partly due to the mesoporous structure of the PbI2 film.The surface morphology of the PbI2 film was revealed by scanning electron microscopy, as shown in Figure S3(c), presenting a mesoporous structure 4 with an average pore size of ~100 nm.Such porosity is also reflected in the measured image of atomic force microscopy, Figure S3(d), exhibiting a root-mean-square roughness of ~7 nm.

Figure
Figure S1 3D diagram of reaction chamber.

Figure
Figure S2 Absorption edge part of a typical absorption spectrum, (), where E is photon energy.

Figure
Figure S7 UV absorption spectra and XRD profiles of MAPbBrxI3-x films grown by immersing PbI2

Figure
Figure S8 Evolution of Br composition  A , extracted from UV-vis optical absorption spectrum

Table S1
Extracted rate constants of PbI2 depletion based on XRD profiles,  PbI 2 's, at different reaction temperatures,  R 's.Table S2 Fitted results of Br composition ( D ) vs. reaction time ( R ) with use of Eq. (7) in the main text.Evolution of MAPbBr x I 3-x Grown with PbI 2 -MABr:MAI Scenario